1. Field of the Invention
The invention relates to an amplifier circuit, and more particularly to a two-order amplifier circuit with high stability and low signal distortion.
2. Description of the Related Art
Along with the developments in portable electronic technology, a variety of related products (such as the cell phone, personal digital assistance, tablet computer, or others) are becoming increasingly diverse. In addition, the multimedia streaming service has become one of the essential functions that are commonly provided by the portable electronic devices. Therefore, a high efficiency and low power consumption power amplifier is highly required by the portable electronic devices. In recent years, the class D amplifier has replaced the class AB amplifier and become a preferred choice as an audio power amplifier due to its merits of having a small circuit area and a 90% high amplifying efficiency. The class D amplifier is especially preferable for small sized portable electronic devices.
The class D amplifier is also called a digital power amplifier, which can output digitalized and amplified signals by modulating and amplifying the input analog signals. FIG. 1 shows a basic circuit diagram of a class D amplifier, in which the input signal Vin is modulated by the PWM (Pulse Width Modulation) modulator as the digital signals. The digital signals are then amplified by the power transistors Q1 and Q2, and the amplified digital signals are filtered by a low pass filter so as to filter out the original input audio signal to be played by the loud speaker.
Because the inputs of the power transistors are digital signals, the power transistors Q1 and Q2 work in the saturated or cut-off regions. Therefore, the power consumption of the power transistors Q1 and Q2 is very small, which may improve the overall efficiency of the power amplifier and reduce the area required by the heat dissipation devices. For these reasons, the circuit area of the class D power amplifier can be greatly reduced. In addition, the amplifying efficiency of a class AB amplifier is only 50%, while the amplifying efficiency of a class D amplifier can be as high as 90%, or even close to 100%. Thus, the class D amplifier has become commonly used in the audio power amplifier field.
Because probable electronic devices are usually used very close to a human body, the Electromagnetic Disturbance (EMI) generated by the probable electronic device must meet statutory standards and should be as small as possible.
Therefore, a two-order amplifier circuit with high stability and low signal distortion, which can reduce EMI and reduce the distortion in the amplified signals, is highly required.